Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography wherein latent electrostatic images formed on a charged retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
Another form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This form of printing differs from the above mentioned xerographic form in that toner is deposited in image configuration directly onto plain paper. The novel feature of DEP printing is to allow simultaneous field imaging and toner transport to produce a visible image on paper directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as it is required in electrophotographic printing.
A DEP printing device has been disclosed in U.S. Pat. No. 3,689,935, issued Sep. 5, 1972 to Pressman et al.
Pressman et al. discloses a multilayered particle flow modulator comprising a continuous layer of conductive material, a segmented layer of conductive material and a layer of insulating material interposed therebetween. An overall applied field projects toner particles through apertures arranged in the modulator whereby the particle stream density is modulated by an internal field applied within each aperture.
A new concept of direct electrostatic printing was introduced in U.S. Pat. No. 5,036,341, granted to Larson, and further developed in a co-pending application.
According to Larson, a uniform electric field is produced between a back electrode and a developer sleeve coated with charged toner particles. A printhead structure, such as a control electrode matrix, is interposed in the electric field and utilized to produce a pattern of electrostatic fields which, due to control in accordance with an image configuration, selectively open or close passages in the printhead structure, thereby permitting or restricting the transport of toner particles from the developer sleeve toward the back electrode. The modulated stream of toner particles allowed to pass through the opened passages impinges upon an image receiving medium, such as paper, interposed between the printhead structure and the back electrode.
According to the above method, a charged toner particle is held on the developer surface by adhesion forces, which are essentially proportional to Q.sup.2 /d.sup.2, where d is the distance between the toner particle and the surface of the developer sleeve, and Q is the particle charge. The electric force required for releasing a toner particle from the sleeve surface is chosen to be sufficiently high to overcome the adhesion forces.
However, due to relatively large variations of the adhesion forces, toner particles exposed to the electric field through an opened passage are neither simultaneously released from the developer surface nor uniformly accelerated toward the back electrode. As a result, the time period from that the first particle is released until all released particles are deposited onto the image receiving medium is relatively long.
As described above, a continuous voltage signal is utilized to modulate a stream of toner particles through a pattern of apertures, opening or closing apertures during predetermined time periods (development periods) during which toner is released from the toner particle source and transported through the opened apertures to the back electrode and the image receiving medium, eg. a paper sheet. When utilizing a continuous control signal (e.g. +300 V) during a specific time period, an amount of particles is released from the particle source and exposed to an attraction force from the back electrode electromagnetic field. Particles which have gained sufficient momentum to pass through the aperture before the end of the time period are still influenced by the control signal even after passage through the aperture and are thus exposed to a divergent electromagnetic field which may cause scattering of the toner particles. On the other hand, particles passing through the aperture immediately after the end of the time period are, during their transport from the aperture towards the back electrode, subjected to a more convergent field which provides more focused transport trajectories and thereby smaller dots. This first state prolongs the time of transport to the back electrode and also elongates the stream of toner particles, which leads to a delayed deposition on the image receiving medium and thus a lower print uniformity at lower speeds. The toner particle stream is also scattered, which results in larger dots and hence a lower print resolution.
This drawback is particularly critical when using dot deflection control. Dot deflection control consists of performing several development steps during each print cycle to increase print resolution. For each development step, the symmetry of the electrostatic field is modified in a specific direction, thereby influencing the transport trajectories of toner particles toward the image receiving medium. This allows several dots to be printed through each single passage during the same print cycle, each deflection direction corresponding to a new dot location. To enhance the efficiency of dot deflection control, it is particularly essential to decrease the toner transport time and to ensure direct transition from one deflection direction to another, without delayed toner deposition. For example, a 600 dpi (dots per inch) deflection control requires a dot diameter below 60 microns and high speed, eg. 10 ppm (pages per minute); dot deflection printing requires shorter toner transport time and faster transition. It is thus essential to ensure that all toner particles released from the toner source are given enough time for all toner particles to be deposited onto the image receiving medium before a transition is made from one deflection direction to another.
Therefore, in order to achieve higher speed printing with improved print uniformity, and in order to improve dot deflection control, there is still a need to improve DEP methods which allows shorter toner transport time and reduces delayed toner deposition and also lowers the consumption of toner per development time unit.
A DEP method in accordance with the above is performed in consecutive print cycles, each of which includes at least one development period t.sub.b and at least one recovery period t.sub.w subsequent to each development period t.sub.b. This DEP method thus includes the steps of:
providing a particle source, a back electrode and a printhead structure positioned therebetween, said printhead structure including an array of control electrodes connected to a control unit; PA1 positioning an image receiving medium between the printhead structure and the back electrode; PA1 producing an electric potential difference between the particle source and the back electrode to apply an electric field which enables the transport of charged toner particles from the particle source toward the back electrode; and PA1 during each development period t.sub.b or selected parts thereof, applying variable electric potentials to the control electrodes to produce a pattern of electrostatic fields which, due to control in accordance with an image configuration, open or close passages/apertures through the printhead structure to selectively permit or restrict the transport of charged particles from the particle source onto the image receiving medium.
When toner particles have passed through an aperture, it has been observed that their transport trajectory is influenced by the control potential of the control electrode pertaining to each aperture. If the control potential is still "ON" (i.e. equals V.sub.on), the toner particle stream tends to be more scattered. If the control potential instead is "OFF" (i.e. equals V.sub.off), the toner particle stream will be more focused.
Thus, the optimal condition is when the control potential is "ON" during the transport of toner particles from the toner particle source, e.g. a developer sleeve, to the printhead structure and immediately switched "OFF" as soon as the toner particles have reached the aperture.